We have discovered a new class of eucaryotic transposable elements, designated FB elements, that are distinct from the standard copia-like elements in several ways. They carry large inverted terminal repeats instead of large direct terminal repeats, they are extremely heterogeneous in construction instead of closely conserved, and they display a most unusual arrangement of short repeated sequences within their large inverted termini. We propose to further investigate the function, structure and unbiguity of this class of mobile elements. Sequence analyses of a small number of FB elements will allow a finer definition of their structure, coding capacity, and the mechanism of generation of "solo" terminal elements. Moreover, we will investigate their specificity and rate of transposition by using in situ hybridization, and nucleotide sequencing of target sites. Preliminary evidence also suggests that the FB elements might be induced to move rapidly by using the IR system of hybrid dysgenesis (W. Gehring, personal communication). Using S1 nuclease we will analyze the structure of the palindromic sequences in nuclei, to determine if they can assume the cruciform configuration, which might play a role in protein binding and/or gene activation. Finally, we will search the Drosophila and human genomes for other FB-like families of transposable elements.